1. Field of the Invention
The present invention relates to a method of producing sintered articles of polybenzimidazole resins.
2. Background Art
Polybenzimidazoles are known as polymers having wide working temperature ranges, excellent in such properties as mechanical strength, chemical resistance, solvent resistance, radiation resistance and flame retardance. A known method of producing a sintered article of polybenzimidazole by sintering polybenzimidazole is such that a mixture of a polybenzimidazole polymer and a polybenzimidazole prepolymer that serves as a sintering aid is sintered with application of sufficiently high heat and pressure (U.S. Pat. No. 3,340,325).
On the other hand, it has been found that sintered articles of polybenzimidazole resins produced by the method described in the specification of U.S. Pat. No. 4,814,530 are superior to those produced by the above-described method in physical properties, and also in the ability to give thick products. The following is a brief description of the method of U.S. Pat. No. 4,814,530.
Namely, a sintered article of polybenzimidazole can be obtained through the steps as described below. A granular polybenzimidazole resin having a particle diameter of not more than 100 meshes, a water and volatile content of not more than 0.1% by weight, and an intrinsic viscosity of at least 0.4 is placed in a mold, and pressed at a pressure of 140.6 to 1406 kg/cm2 at room temperature for at least 1 minute. While maintaining the pressure in this range, the pressed polybenzimidazole resin is heated to a predetermined temperature ranging from 441xc2x0 C. to 510xc2x0 C. (heating step). After the temperature of the polybenzimidazole resin has reached the predetermined temperature, the pressure on the resin is released, and the resin is heated at the temperature or higher for 4 hours (sintering step). Thereafter, the sintered article of the polybenzimidazole resin obtained is pressed again at a pressure in the above-described range, and cooled to 427xc2x0 C. or lower; the cooled article is then held at a temperature ranging from 441xc2x0 C. to 510xc2x0 C. at the pressure for at least 1 hour (post cure step), thereby finally obtaining a desired sintered article of the polybenzimidazole resin.
Japanese Laid-Open Patent Publication No. 156847/1998 proposes a method of producing a porous-part-free sintered article of polybenzimidazole, capable of minimizing the formation of voids that occurs due to gases emitted by the thermal decomposition of polybenzimidazole, and also preventing the expansion of the sintered article itself that takes place when the pressure on the mold is released (spring back phenomenon).
The above-described method is specifically as follows: while fixing the platen of a pressing machine at such a position that a polybenzimidazole resin, starting material, can be densely packed in a mold placed under the platen, that is, without applying pressure to the mold from the outside, the resin is heated to a predetermined temperature, that is, a sintering temperature; and after the temperature of the resin has reached the sintering temperature, a predetermined pressure is applied to the mold. By this method, it is possible to prevent the formation of voids in the sintered article, and also the expansion of the sintered article that is caused by the spring back phenomenon. Since the thermal conductivity of polybenzimidazole resins is low, it takes time, in this method, to elevate the temperature of the resin to the predetermined one. To compensate for this shortcoming, the polybenzimidazole resin may be sintered at higher temperatures. In this case, however, the polybenzimidazole resin can be partially oxidized, and the oxidized part of the resin can have decreased strength. Therefore, the operation under high temperatures requires great care.
To overcome the aforementioned drawbacks in the prior art, there has been demanded a method of producing, in high yields and productivity, sintered articles of polybenzimidazole resins in desired shapes, having few low-strength parts such as voids.
We found that it is possible to shorten the cycle time, and, at the same time, to maintain the product quality by preventing polybenzimidazole resins from coming into contact with oxygen while they are subjected to high temperature conditions.
Namely, a method of producing a sintered article of polybenzimidazole according to the present invention comprises, in the following order, the steps of:
(1) filling a mold with a polybenzimidazole resin,
(2) after closing the mold so that the polybenzimidazole resin can be densely packed in the mold, heating the mold to a predetermined temperature ranging from 500xc2x0 C. to 600xc2x0 C. without applying pressure to the mold from the outside,
(3) after maintaining the temperature for 0 to 100 minutes, and then increasing the pressure on the mold from 0 kg/cm2 to 50-750 kg/cm2, sintering the polybenzimidazole resin while maintaining the temperature and the pressure for 15 to 200 minutes,
(4) cooling the mold to a temperature ranging from 50xc2x0 C. to 400xc2x0 C., and
(5) removing the sintered article of the polybenzimidazole resin from the mold, wherein the polybenzimidazole resin is brought into contact with substantially no oxygen at least in step (2) and step (3).
According to the above-described method of the invention, it is possible to produce sintered articles of polybenzimidazole resins having quality comparable to that of sintered articles obtainable by the conventional methods, by using lower pressure conditions and shorter cycle times than those in the conventional methods.